1. Technical Field
The present invention relates to a fuel cell power generation system which directly converts chemical energy of a fuel into electrical energy, and more particularly to a method of controlling a differential pressure of a plate reformer which is installed in the power generation system to reform the raw material to a fuel gas and feed the fuel gas into an anode of a fuel cell.
2. Background Art
Among various types of fuel cell power generation system, one of typical power generation systems incorporating a molten carbonate fuel cell utilizes a natural gas as a raw material gas. A fundamental structure of such a fuel cell power generation system will be described with reference to FIG. 4 of the accompanying drawings.
A fuel cell FC is a stack fuel cell elements which are piled up with separators being interposed. Each cell element includes an electrolyte plate (tile) 1, a cathode (electrode) 2 and an anode (electrode) 3, and the tile 1 is sandwiched by these electrodes 2 and 3. An air line 4 extends to the cathode 2 and an air A pressurized by a compressor 5 is introduced to the cathode 2 via an air preheater 6 through the air line 4. Gases discharged from the cathode 2 (called "cathode exhaust gas") flow in the air preheater 6 and a steam generator (not shown) before being expelled to the atmosphere.
A natural gas NG is used as a raw material gas in this power generation system. The natural gas NG is desulfurized by a desulfurizer (not shown) and mixed with a steam H.sub.2 O. This mixture is introduced to a natural gas feed line 7 and flows in a natural gas preheater 8 and a reforming chamber 9a of a reformer 9 in turn. The natural gas becomes a fuel gas as it is reformed in the reforming chamber 9a. The fuel gas flows in a fuel gas feed line 10 and the natural gas preheater 8 before it reaches the anode 3. Gases discharged from the anode 3 (called "anode exhaust gas") are introduced to a combustion chamber 9b of the reformer 9 by an anode exhaust gas line 11. In the combustion chamber 9b, combustible components in the anode exhaust gas are burned with the air fed from a line 12 branched from the air line 4. A combustion exhaust gas which contains CO.sub.2 and is discharged from the combustion chamber 9b proceeds to a heat exchanger 13 and a recycle blower 14. The exhaust gas is pressurized by the blower 14 and supplied to the cathode 2 together with the air. "M" indicates a motor.
Flow rate control valves 15 and 17 are provided on the air feed line 4 and the natural gas feed line 7, respectively. These valves 15 and 17 are connected with flow rate controllers 16 and 18, respectively, and these controllers are connected with a master controller (not shown) such that the air flow rate and the fuel flow rate are adjusted in accordance with a required output of the fuel cell FC.
Recently, a plate reformer is used as the reformer 9 since the plate type one is compact and uniform combustion, which in turn results in effective reformation, can be expected in all over the combustion chamber 9b.
A conventional plate reformer is categorized into two types: air dispersion type and fuel dispersion type. An air is dispersed in the fuel in the former type and a fuel is dispersed in an air in the latter type. One example of the former type is disclosed in Japanese Utility Model Application, Publication No. 2-37739 and that of the latter type is disclosed in Japanese Utility Model Registration No. 1,952,542. As illustrated in FIG. 5 of the accompanying drawings, the plate reformer of the air dispersion type includes a plurality of reforming chamber 9a and combustion chambers 9b piled up with heat transfer walls 21. A reforming catalyst 19 is placed in the reforming chamber 9a. The combustion chamber 9b is divided into two sub-chambers (a catalyst chamber 20b and an air dispersion chamber 23) by an air dispersion plate 22. The air dispersion plate 22 has a number of dispersion openings. A combustion catalyst 20 is placed in the catalyst chamber 20b. An air A is fed to the catalyst; chamber 20b from the air dispersion chamber 23 through the openings of the air dispersion plate 22. The reforming chamber 9a and the combustion chamber 9b are shaped like plates, respectively. These chambers are piled up and welded to each other at their peripheries to form a single plate reformer of air dispersion type. The fuel F is fed to the catalyst chamber 20b of the combustion chamber 9b and the air A is fed to the catalyst chamber 20b from the air dispersion chamber 23 via the dispersion plate 22 so that the air A disperses within the fuel F in the catalyst chamber 20b while being combusted. Then, a combustion exhaust gas CG is discharged from the catalyst chamber 20b (or the combustion chamber 9b). Heat generated upon combustion in the combustion chamber 9b is transferred to the reforming chamber 9a via the heat transfer wall 21 and used as a heat source for the reforming reaction. The raw material gas NG is reformed in the reforming chamber 9a and discharged as a reformed gas
A plate reformer of fuel dispersion type is obtained by feeding the fuel F to the dispersion chamber 23 and feeding the air A to the catalyst chamber 20b.
The plate reformer 9 installed in the above-described fuel cell power generation system includes a plurality of flat elements joined with each other by the welding so that if a differential pressure between the reforming chamber 9a and the combustion chamber 9b and that between the inside of the reformer 9 and the outside of the same become too large, the welding cannot bear the differential pressure. Therefore, the differential pressures should be suppressed under prescribed values, respectively.
One pressure difference controlling method is schematically illustrated in FIG. 6 of the accompanying drawings. This method may be applied to controlling the pressure difference between the reforming chamber 9a and the combustion chamber 9b and that between the inside and outside of the reformer 9. As shown in FIG. 6, the plate reformer 9 is placed in a pressure vessel 24, a differential pressure meter 25 is provided between an exit of the reformer chamber 9a and an entrance of the combustion chamber 9b, another differential pressure meter 26 is provided between an entrance of the reforming chamber 9a and an exit of the combustion chamber 9b, a differential pressure control valve 27 is provided to adjust a flow rate of a gas from the exit of the reforming cabinet 9a upon receiving an instruction from the differential pressure meter 25, and another differential pressure control valve 28 is provided to adjust a flow rate of a combustion exhaust gas upon receiving an instruction from the differential pressure meter 26. In addition, differential pressure control valves 31 and 32 are provided on a fluid feed line 29 and a fluid discharge line 30 to control a pressure inside the pressure vessel 24. Further, a differential pressure meter 33 is provided to detect a differential pressure between the entrance of the combustion chamber 9b and the inside of the pressure vessel 24, and another differential pressure meter 34 is provided to detect a differential pressure between the exit of the combustion chamber 9b and the inside of the pressure vessel 24. The differential pressure control value 31 is controlled by the differential pressure meter 33 and the differential pressure control valve 32 is controlled by the differential pressure meter 34.
However, the above-described conventional method using the differential pressure control valves cannot insure that the differential pressure always stays in a predetermined range when a trouble occurs in the valves 27, 28, 31 and 32, the differential pressure meters 25, 26, 33 and 34, an air feed line, a power source or other components. Therefore, the conventional method has a problem in reliability and a large expense should be used to these components.
Another differential pressure method is known which is applied to controlling of a differential pressure between the anode and cathode of the fuel cell: the anode exhaust gas and the cathode exhaust gas are both introduced to a catalyst combustor and mixed therein so that the gas pressure of these gases made equal to each other. However, it is difficult to simultaneously control a differential pressure between the reforming chamber and combustion chamber of the plate reformer.